1. Field of the Invention
The invention relates to a transmission system for producing a replica of a wideband digital signal which includes at least a first and a second component; and more particularly to such a system which comprises an encoder including an analyzer for altering the digital signal to obtain a number n of sub-signals for said digital signal; a transmitter for transmitting the sub-signals for reception at a different time or place; a receiver for receiving the sub-signals; and a decoder including a synthesizer for combining the received sub-signals to obtain respective replicas of the digital signal. The invention also relates to an encoding transmitter, and a decoding receiver for such a system.
2. Description of the Prior Art
One transmission system of this type is known from an article, "The Critical Band Coder--Digital Encoding of Speech Signals Based on the Perceptual Requirement of the Auditory System" by M. E. Krasner, published in Proc. IEEE ICASSP 80, Vol. 1, pp 327-331, Apr. 9-11, 1980. This article relates to a transmission system in which the sub-signals are signals representing frequency bands. The transmitter includes a frequency subband coding system in which the speech signal band is divided into a plurality of subbands whose bandwidths approximate the bandwidths of the critical bands of the human ear in the respective frequency ranges (see FIG. 2 of this article). This division is selected because, based on psycho-acoustic experiments, one can expect that quantization noise in such a subband will be masked to an optimum extent by the signals in that subband, if during quantization allowance is made for the noise-masking curve of the human ear. Threshold values for noise masking by single tones are shown in FIG. 3 of this article. The receiver employs a corresponding subband decoding system.
When applying frequency subband coding to a high-quality digital music signal, such as one according to the Compact Disc Standard which uses 16 bits per signal sample at a sample frequency of 1/T=44.1 kHz, with a suitably selected bandwidth and a suitable selected quantization for the respective subbbands, the quantized output signals of the coder can be represented by an average number of approximately 2.5 bits per signal sample. The quality of the replica of the music signal does not differ perceptibly from that of the original music signal in substantially all passages of substantially all kinds of music signals.
The subbands need not necessarily correspond to the bandwidths of the critical bands of the human ear. For example, the subbands may have equal bandwidths, provided that allowance is made for this in determining the masking threshold.
The invention is also applicable to other types of transmission systems, such as those in which blocks of samples are transform coded. Such systems are referred to in the article "Low bit-rate coding of high-quality audio signals. An introduction to the MASCAM system" by G. Thiele, G. Stoll and M. Link, published in EBU Technical Review, no. 230, pp. 71-94, August 1988. In such a system the transform coefficients correspond to the sub-signals.
The sub-signal transmission systems described above have the disadvantage that, in some cases, perceptible differences occur between the replica and the signal which was to be transmitted. These differences are perceived as a form of distortion in the replica generated by the receiver. Often they are the result of the number of bits, available for quantization of certain of the sub-signals, being too low.